In general, the voltage delivered by the battery of a motor vehicle is liable to vary, and runs the risk of damaging the electric members of the power supply network. It is then necessary to provide a device for generating a regulated voltage from the voltage of the source.
A known device comprises a “chopper” or switch mode power supply (SMPS) of the type having a “flyback” converter.
In conventional manner, such a device comprises:                an energy-storage transformer having magnetically coupled primary and secondary windings;        a primary circuit connecting the power supply source to a ground, and comprising the primary winding and a static main switch connected in series; and        a secondary winding designed to be connected to a load, and including the secondary winding.        
In general, such a device operates cyclically at a predetermined frequency referred to as the switching frequency. In particular, each cycle comprises two stages: a stage of storing magnetic energy, also referred to as the “inlet” stage, during which the main switch is in the closed state; and a state of delivering magnetic energy, also referred to as the “outlet” stage, during which the main switch is in the open state.
The secondary circuit generally comprises a rectifier element connected in series with the secondary winding, and a capacitor element connected in parallel with the secondary winding and the rectifier element, and designed to deliver the regulated output voltage across its terminals.
During the input stage, closure of the main switch allows the input voltage to be applied to the primary winding, thereby causing a primary current to appear and thus storing magnetic energy in the transformer. The rectifier element is in the off state and then prevents current from flowing through the secondary winding, and the capacitor element discharges through the load.
During the output stage, opening the main switch prevents the primary current from flowing in the primary circuit. Conservation of the magnetic energy stored in the transformer then causes a secondary current to appear that flows in the flow direction of the rectifier element in the secondary circuit, thereby enabling the capacitor element to be charged. The magnitude of the current flowing in the secondary circuit decreases until there is no more magnetic energy in the transformer.
In order to control the opening and closing of the main switch in cyclical manner, the device includes voltage control means for the static main switch.
Such control means generally comprise a static control switch connecting the control terminal of the static main switch to ground and a feedback circuit connecting the control terminal of the main static switch to the secondary circuit, the feedback circuit then being activated on opening of the main switch. The feedback circuit serves in particular to guarantee that the main switch opens fully.
When the magnitude of the primary current reaches a predetermined threshold value, the static control switch switches from the open state to the closed state, thereby causing the control terminal of the main switch to be connected to ground. This causes the main switch to open and activates the feedback circuit. The main switch then remains open until the magnitude of the secondary current becomes zero as a result of the magnetic energy stored in the transformer being used up, thereby causing the main switch to close immediately.